Get the average and peak intensity (sum of r/g/b channels) of the centre 100x100px of an image.

jpeg_intensity.go

package main

import (
	"os"
	"fmt"
	"image/jpeg"
	"bufio"
	"log"
)

func main() {

	if len(os.Args) < 2 {
		log.Fatal("Supply filename as argument")
	}

	var fname = os.Args[1]

	// open file
	f,err := os.Open(fname)
	if err != nil {
		log.Fatal("File:", fname, "Error:", err)
	}
	defer f.Close()
	fb := bufio.NewReader(f)

	// decode
	img, err := jpeg.Decode(fb)
	if err != nil {
		log.Fatal("File:", fname, "Error:", err)	
	}

	// iterate over pixels, using a square of 100x100 in the middle
	const SampleSize=100
	r := img.Bounds()
	var totalPixels, peakR, peakG, peakB, avg uint32
	sizeX := r.Max.X - r.Min.X
	sizeY := r.Max.Y - r.Min.Y
	for x:=(sizeX/2)-(SampleSize/2); x < (sizeX/2)+(SampleSize/2); x++ {
		for y:=(sizeY/2)-(SampleSize/2); y < (sizeY/2)+(SampleSize/2); y++ {
			colour := img.At(x,y)
			r,b,g,_ := colour.RGBA()
			if r > peakR {
				peakR = r
			}
			if g > peakG {
				peakG = g
			}
			if b > peakB {
				peakB = b
			}
			sum := r+b+g
			avg += sum
			totalPixels += 1
		}
	}
	avg /= (totalPixels * 3)

	// print result
	fmt.Println("Max possible", 0xFFFF, "over", totalPixels, "pixels")
	fmt.Println("Average:", avg)
	fmt.Println("Peaks:", peakR, peakG, peakB)
}